As illustrated in FIG. 1, an electrophotographic type image forming apparatus includes a Beam Detect (BD) sensor on the starting side of a light beam scanning area. The BD sensor is configured to, as a light beam for scanning by a light beam scan unit passes therethrough, output a detection signal of the magnitude corresponding to the amount of received light of the light beam. The light beam scan unit controls a timing to write an electrostatic latent image on a photoreceptor drum in accordance with the detection signal of the BD sensor.
For instance, the light beam scan unit processes a detection signal of the BD sensor with a light beam detection circuit as shown in FIG. 2A. This light beam detection circuit includes a comparator having an inverting input terminal that receives, as an input, a detection signal (voltage V−) of the BD sensor subjected to current-voltage conversion. The comparator has a input terminal that receives, as an input, predetermined voltage V+ as a threshold level (threshold value). As shown in FIG. 2B, the comparator outputs a synchronizing signal when the voltage V− exceeds the voltage V+. Upon detecting a change of the synchronizing signal of the comparator from H level to L level, a control section sets a starting position to write an image.
Some conventional electrophotographic type image forming apparatuses are adapted to change the light amount of a light beam depending on ambient temperature or ambient humidity of the apparatus, a deterioration state of the photoreceptor drums and the like (see Patent Literature 1, for example). Such an image forming apparatus, however, has a problem that a change in the light amount of a light beam causes displacement of a starting position to write an image. For instance, as shown in FIG. 2C, since the magnitude and the tilt of a detection signal (voltage) of the BD sensor differ between the amount of received light of a light beam of 220 μW and 80 μW, the detection timing of the light beams will be different therebetween by about 660 ns. At this time, when the light beam scan speed by the light beam scan unit is 1,612,149 mm/s, then the starting position to write an image will be displaced by as large as about 1.1 mm.
Then, a light scanning recorder conventionally disclosed includes two optical sensors disposed adjacently and compares waveforms output from the two optical sensors, thereby enabling detection of a passage timing of a light beam always precisely without displacement (see Patent Literature 2, for example).